1. Field of the Invention
The present invention is directed to a diaphragm arrangement for opto-electronic scanning of originals according to the preamble of claim 1.
2. Description of the Related Art
In opto-electronic scanning of originals as employed, for example, in reproduction technology in what are referred to as scanners, an original is scanned point-by-point and line-by-line, whether opaquely or in transparency. The scanning light ray is forwarded via a diaphragm arrangement onto one or more optoelectronic transducers at whose output the image signals appear in analog form. Such scanners are used in reproduction for scanning black-and-white originals or chromatic originals, whereby either color separations for multi-color printing (chromatic printing) or color images are produced in color reproduction.
U.S. Pat. No. 2,691,696 shows a number of embodiments of such apparatus. In order to improve the image sharpness, what is referred to as "unsharp masking" is used in this patent, whereby a sharp and an unsharp scanning of the image are undertaken and the image signals thereby acquired are electronically united to form a recording signal.
In an embodiment disclosed by U.S. Pat. No. 2,691,696, two light sources are used at the scanning side for contrast enhancement (unsharp masking), whereby the one light source is provided for the sharp scanning (sharp channel) and the second light source is provided for the unsharp scanning (unsharp channel). The size of the sharp scanning spot and of the unsharp scanning spot is respectively defined by the aperture of two diaphragms that are respectively arranged in the corresponding light rays before the light rays impinge the scan location of the original. The signal separation of sharp signal and unsharp signal is undertaken either by separate opto-electronic transducers or mechanically (light chopper).
In another embodiment of U.S. Pat. No. 2,691,696 (FIG. 15), only one light ray and only one opto-electronic transducer are used at the scan side. Before the light impinges the scanning original, the light source is imaged with two optics onto two dynamically operating light valves arranged at a distance from one another. These light valves are electrically controllable and generate a variable light slit whose width depends on the electrically controlled signal. The light valves that are respectively arranged in the focal points of the optics lie at a right angle relative to one another and their interaction yields a quadratic or rectangular aperture that is imaged with a third optics onto the scan location of the original. The light the original allows to pass proceeds onto an opto-electronic transducer whose output signal is alternately switched onto two signal channels in a defined clock. The two light valves are likewise controlled such with this switching clock that the slit widths respectively assume two values in alternation, namely narrow or broad. When both light valves are at "narrow", then the transducer outputs an electrical signal for the sharp scanning (sharp channel); when both light valves are at "broad", then the signal for the unsharp scanning (unsharp channel) is taken at the transducer.
A sharp signal and, respectively, unsharp signal can in fact be alternately acquired in chronological succession with such an arrangement; however, the light valves are relatively slow, this leading to a low scanning rate.
Since, however, high scanning speeds are a matter of concern in such scanners, it is necessary that both the sharp signal as well as the unsharp signal are constantly adjacent without interruption, for which reason these scanning units having chronologically interrupted scan signals of low frequency have not prevailed in practice.
Scanner arrangements comprising diaphragm arrangements have therefore been utilized that work with high scanning frequency and wherein the scan signals are supplied without interruption. Such diaphragm arrangements that have been utilized both for the unsharp masking as well as for scanning originals that are already screened comprise a first diaphragm, also referred to as main diaphragm, that is arranged within the scanning light beam reflected by the original or allowed to pass by the original. The aperture of this diaphragm allows a central region of the scanning light beam to pass (main field), this being forwarded onto a first opto-electronic transducer that supplies the actual scanning image signal in the scanning spot.
A second diaphragm having a larger diaphragm aperture than the main diaphragm (also referred to as surrounding field diaphragm) is also provided, this extracting a sub-beam from the scanning light beam in combination with a partially transmitting mirror. A larger region of the original surrounding the sharp scanning spot, the size thereof being defined by the aperture of this surrounding field diaphragm, is extracted by this diaphragm and is forwarded onto a second opto-electronic transducer that supplies what is referred to as the surrounding field signal.
German Pat. No. 30 10 880 shows such a diaphragm arrangement wherein that side of the main diaphragm facing toward the scanning light beam comprises a mirroring in the region of the diaphragm aperture, a part of the scanning light beam (surrounding field) being mirrored out by this mirroring and being forwarded onto the second photo-electrical transducer via the surrounding field diaphragm. The surrounding field signal is electronically further-processed together with the main signal, this being disclosed, for example, in German Pat. No. 10 39 842.
The advantage of this diaphragm arrangement for scanning originals lies therein that, in comparison to U.S. Pat. No. 2,691,696, the scan signals of the main field and surrounding field are constantly supplied in parallel, this allowing a fast scanning and signal processing. Further, an exact separation of main field signal and surrounding field signal is established, this not being the case in U.S. Pat. No. 2,691,696. In the U.S. Patent, the signal of the sharp channel corresponds to the main field signal of German Pat. No. 30 10 880, but the signal of the unsharp channel does not correspond to the surrounding field signal; rather, the unsharp signal also contains the modulation of the sharp, main picture element since the diaphragm in the unsharp channel merely scans a larger spot than the diaphragm in the sharp channel.
The surrounding field signal that is supplied by the reflecting diaphragm of German Pat. No. 30 10 880 contains only the signal components of the surrounding field surrounding the sharp picture element.
Since it is not only the unsharp masking or, when scanning rastered originals, a de-rastering that is to be undertaken with the diaphragm arrangement in such scanners but there is usually also the demand for modification of the reproduction scale between original and recording, it is necessary to undertake the scanning with different scanning fineness depending on the scale, to which end the size of the scan spot, i.e. of the diameter of the main diaphragm, as well as the outer limitation of the surrounding field by the aperture of the surrounding field diaphragm must be adapted to the scanning fineness dependent on the scanning fineness selected.
It would be fundamentally possible to undertake this adaptation with the light valves of U.S. Pat. No. 2,691,696; however, as already mentioned, a low scanning frequency, i.e. slow scanning, would have to be accepted. On the other hand, a clean signal separation between scanning spot and the signal that derives from the surrounding field surrounding the scanning spot would not be obtained.
Moreover, the utilization of such light valves of U.S. Pat. No. 2,691,696 in an arrangement of German Pat. No. 30 10 880 is not possible since such a light valve enables a different adjustment of a light slit in only one dimension and two such light valves cannot be spatially arranged at one location. We would like to reference U.S. Pat. No. 3,646,262 in this context wherein such a light valve is shown in FIGS. 10 through 12.
A further disadvantage of such light valves is comprised therein that it is not possible to separately extract the surrounding field with such light valves.
Adjustable diaphragms, for example iris diaphragms that are composed of a plurality of lamellae are also known, these being correspondingly turned or, respectively, displaced for the adjustment of the diaphragm. The utilization of such diaphragms in a diaphragm arrangement of German Pat. No. 30 10 880 is likewise not possible since, even if the back side of the lamellae is mirrored, a planar mirror cannot be produced with which an exact optical imaging is possible. One reason lies in the differing thickness of the lamellae and in the fact that the lamella thicknesses cannot be arbitrarily reduced for obtaining the required, mechanical stiffness. Further, the flexibility of the lamellae prevents an exact optical imaging. These diaphragms are thus eliminated for utilization as reflecting diaphragms.
For adapting the diaphragm diameter to the different scanning finenesses, a plurality of diaphragms differing in diameter and allocated to one another are therefore used in practice, these, for example, being arranged in what is referred to as a diaphragm wheel. Dependent on the scanning fineness selected, two different diaphragms adapted to one another are swivelled into the scanning light beam. Since, however, there is only one optimum main diaphragm diameter for every scanning fineness, this determining the size of the scanning light spot, it is clear that an optimum scanning can only ensue for these individual diaphragms even given a plurality of diaphragms. A mismatch of the diaphragm is established for scanning finenesses that lie between these diaphragm diameters and this must in fact be accepted. A loss of contrast occurs given too large a diaphragm (over-scanning) and scanning gaps occur given too small a diaphragm (under-scanning). An information loss, i.e. a quality loss, is unavoidable in both cases. An apparatus that is provided with such a diaphragm wheel is described, for example, in the operating instruction, Vario-Klischograph K 181, Edition 3, December 1963, of Dr.-Ing. Rudolf Hell GmbH, Kiel.
In order to counteract the disadvantages occurring upon utilization of diaphragm wheels EP-A-1 0 077 410 recites a method for improving the contrast enhancement given variable reproduction scale wherein a designational modification of the transmission width of correction signal (surrounding field signal) and image signal (main field signal) is set dependent on the scale selected. This in fact makes an electronic attempt to undertake a matching of the contrast enhancement to the different scanning resolution; however, an optimum diaphragm adaptation of both the main diaphragm as well as of the surrounding field diaphragm does not occur, so that over-scanning or, respectively, under-scanning also occur here.
As may be seen therefrom, one must therefore fundamentally count on a quality decrease given scanning finenesses that are not optimized to the respective diaphragm diameter.
When rastered originals are scanned, a residual moire occurs in addition to a quality decrease since filtering the raster frequency of the raster of the rastered original out can likewise ensue only with diaphragm diameters optimally adapted to the raster of the original, this being disclosed, for example, by EP-A-2 0 065 281.